8/16/2010 - 8/22/2010

Nitrogen (N) Response Trials AddedThe Iowa nitrogen (N) response database in the corn nitrogen rate calculator was recently updated, with response trials added from 2009 research. There are now 188 trials for corn following soybean and 89 trials for corn following corn. Being able to easily update the database with recent data is one of the many advantages to this dynamic database approach for corn N rate guidelines. Having new response trial data allows rapid updating with changing hybrid genetics, rotations and climatic conditions.

With the updated database, calculated N rates have changed slightly from last year. The table below gives the N rate at the maximum return to N (MRTN) and the profitable N rate range from the updated calculator for several N:corn grain price ratios. You can work with any price of N and corn you wish when running the calculator. Output information includes the N rate at the MRTN, the profitable N rate range, the net return to N application, the percent of maximum yield and the selected N fertilizer product rate and cost.

What is the Corn Nitrogen Rate Calculator?The Corn Nitrogen Rate Calculator Web tool is located at http://extension.agron.iastate.edu/soilfertility/nrate.aspx. It is a resource that aids N rate decisions for corn production and is helpful in determining the effect of fertilizer and corn price on application rates. The method for calculating suggested N rates is based on a regional (Corn Belt) approach to N rate guidelines. Details on the approach are provided in the regional publication Concepts and Rationale for Regional Nitrogen Rate Guidelines for Corn, PM 2015. This approach and the Corn Nitrogen Rate Calculator are now being used by seven states across the Corn Belt: Iowa, Illinois, Indiana, Michigan, Minnesota, Ohio and Wisconsin.

Stink bugs are often more prevalent in soybean fields around this time of year, especially in the southern half of the state. Stink bug nymphs and adults are fluid feeders with piercing-sucking mouthparts. Most species are herbivores but a few are beneficial predators. Because herbivorous stink bugs build up in soybean after bloom, they can be found feeding on developing pods, resulting in shriveled, desiccated seeds. The species below are known to occasionally cause problems for Iowa producers.

Green stink bug (Acrosternum hilare)This pest migrates up from southern states each summer and has only one generation per year in Iowa. They usually begin feeding in soybean in early August. Green stink bug nymphs are black with bright green and yellow or red markings. Adults are light green, shield shaped, and have fully developed wings. They are large, about 5/8 inch long.

Brown stink bug (Euschistus servus)These nymphs are light greenish brown and will develop brown spots on the middle of the abdomen as they get older. As an adult, the brown stink bug is brown with slightly rounded shoulders. The underside of their abdomen is yellow to light green. Brown stink bug nymph and adult.Photos by Marlin E. Rice, Iowa State University.

In addition to these well-known species, several economically important stink bugs from other regions are now entering the Cornbelt. None have been documented causing serious damage in Iowa, but these species are on the move. Scout carefully!

Brown marmorated stink bug (Halyomorpha halys)Originally from Asia, this pest was first found in Pennsylvania in 1998. Since then, it has been identified in many states in the continental U.S., including Iowa. “Marmorated” refers to their marble-like coloration. Adults have light bands on the antennae and dark bands on the membranous part of the wings. Additionally, they have small, round coppery or metallic blue depressions on the head and pronotum. Adults are about 2/3 inch long. Nymphs have a yellowish to off-white abdomen, and at the last stage before adulthood, the abdomen will have reddish spots. Their eyes are deep red.

Red banded stink bug (Piezodorus guildinii)This pest has been observed in Arkansas, Louisiana, Mississippi, Missouri, and Tennessee and has spread quickly throughout these states. It has presented management challenges, especially in Louisiana, and has a lower economic threshold than the brown stink bug. Nymphs are mostly green with red and black markings. Adults are green and just under ½ inch long; they can be identified by the two strips (one yellow and one dark red, purple or black) across the “shoulders.” When flipped on its back, a spine is visible on the red banded stink abdomen between the third pair of legs.

Red shouldered stink bug (Thyanta custator custator)This pest has also been reported in several states but is somewhat easier to manage than the red banded stink bug. Adults are green and just under ½ inch long; they have markings that are very similar to the red banded stink bug. However, when flipped over, there is no spine on the abdomen.

Remember, not all stink bugs found in soybean are pests. Some are beneficial and will use their mouthparts to pierce the body and suck out the internal fluids of a variety of soybean pests, particularly caterpillars, beetle larvae and adult beetles.

Spined soldier bug (Podisus maculiventris)The most common beneficial stink bug in Iowa soybean is the spined soldier bug. Both the immature and adult stink bugs of this species are predatory and will kill many soybean pests, including green cloverworm. The spined soldier bug adult is easily identified by its sharply pointed “shoulders” and dark brown spot on the wing tips.

Scouting and Economic Thresholds for Stink BugsTo scout for stink bugs, use sweep nets or shake plants over a drop cloth. Adults tend to aggregate, so sample several areas in each field. Combine counts of older nymphs (larger than 1/4 inch in length) and adults. Economic thresholds for each species are listed in Table 1.

Rebekah Ritson is an entomology research graduate assistant; she can be reached at rritson@iastate.edu or 515-294-1999. Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at ewh@iastate.edu or phone 515-294-2847.

As soybean fields enter seed set, growers are getting concerned about late-season soybean aphid infestations. So far, much of the state has experienced very low aphid populations. Some fields in northern Iowa exceeded the economic threshold and were treated last week. But in general, most fields have only patchy aphid colonies well below the threshold.

There is much evidence to show treating aphids when they exceed the economic threshold (250 aphids per plant with increasing populations on 80 percent of the plants) up to R5.5 will protect yield. But what happens if aphid populations are still increasing past seed set? This was the trend in 2008 when aphid populations were still increasing into September. So do late season insecticides financially make sense for managing soybean aphid? This is a difficult topic for us to discuss because of the lack of replicated data throughout the North Central Region.

A previous article in ICM News showed results from one small plot study at Nashua, Iowa. These data show a R6 treatment may not be worth it. Plants at R6 and beyond may be able to tolerate more aphids without experiencing yield loss. Unfortunately, we do not have a robust data set to help us make recommendations at R6 like we do for aphid outbreaks that occur during R1-R5. So as fields enter seed fill and begin to mature, consider the following factors before treating for soybean aphid regardless of plant stage. Are numbers going up and how fast? Careful field monitoring throughout the summer should give you an indication of the aphid trajectory. Is the value of the insecticide application worth its cost? Depending on the delivery method (ground or aerial) and product choice, control costs can exceed $15 per acre. Keep in mind treating with ground equipment after the canopy closes can reduce yield by one to two bushels per acre.

If you decide to treat for aphids, remember to use sufficient volume and pressure to make contact with aphids on the undersides of leaves and in the lower canopy. This is especially true for beans late in the season when canopies have closed. Whenever possible, leave a check strip so that you can evaluation the value and performance of your insecticide application at harvest.

Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at ewh@iastate.edu or phone 515-294-2847. Matt O'Neal is an assistant professor of entomology with teaching and research responsibilities. He can be reached at oneal@iastate.edu or at 515-294-8622.

This article was published originally on 8/23/2010 The information contained within the article may or may not be up to date depending on when you are accessing the information.

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